Polymer rod suspension insulators, like all rod type insulators, suffer from nonlinear voltage distribution along their lengths to an even greater extent than insulators which are broken up with intervening metal portions, such as conventional ceramic suspension insulators. This nonlinear distribution puts excessive voltage stresses in the areas near the metal fittings of the insulator, particularly at the energized end. As the voltage class of the insulator increases, the stresses become greater. Eventually, there may be a breakdown of the air in the high stress area, creating corona and radio noise.
With polymer insulators, the phenomenon occurs with units energized on system voltages above 138 kV. Up through 138 kV system voltage, the standard end fitting is sufficient to maintain radio influence voltage (radio noise) values below 100 .mu.V at the nominal operating line to ground voltage or even 10% or more above nominal.
However, at system voltages of 161 kV and above, excessive radio noise is generated and the standard design cannot be used. There is needed some simple and effective way to reduce the voltage stress to levels where it will not cause visible corona and excessive radio noise.
This reduction of voltage stress may be accomplished by the well known means of corona rings. However, such rings are expensive, awkward in size and visually obtrusive at such voltages as 161 kV where overhead lines are relatively close to the ground level observer.